Multiple forms of lactic dehydrogenase in tissues of the mouse: their specificity, cellular localization, and response to altered physiological conditions.

There is increasing evidence that many enzymes exist in tissues and cells in multiple molecular forms (Markert and Moller, 1959). Of less certainty, however, is the question of differences between these multiple molecular forms in terms of their catalytic and physiological activities. Indeed Markert and Moller felt that these forms, termed isozymes, possessed identical substrate specificities although the pattern of the isozymes possessed by a tissue changed radically during development. The question of the degree of heterogeneity existing between the multiple molecular forms of an enzyme is one of considerable importance. Such heterogeneity may be reflected in several diverse ways: (1) differences in substrate specificity, (2) dzerences in capacity to utilize cofactor analogs, (3) differences in sensitivity to inhibitors, (4) differences in response to varied physiological conditions, or (5) differences in the site of localization within or between cells. An examination of these possible points of difference between the multiple forms of lactic dehydrogenase in the tissues of the mouse comprises the content of this paper. Department of Zoology, The University of Michigan, Ann Arbor, Mich.